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Teva Pharmaceutical Industries could lose exclusive sales rights to a narcolepsy drug it received when acquiring U.S. company Cephalon in October.

Rival company Mylan has received tentative U.S. Food and Drug Administration approval to sell its generic version of narcolepsy drug Nuvigil, a Cephalon drug that is patent-protected until 2024 (unless Mylan's patent challenge in court is successful ).

Cephalon sued Mylan in the United States and France back in 2010 for alleged patent infringement over Nuvigil.

In any case, Mylan can't start selling its copycat version of the narcolepsy treatment until 30 months from the day Cephalon sued it, or until - if - it wins the patent trial, whichever is sooner. The 30-month count expires in August, which is when Teva could lose exclusivity over marketing the drug if Mylan goes ahead and launches. Theoretically, Mylan is risking a loss in the patent case.

Nuvigil sales totaled $111 million in the first half of 2011. Teva projected sales of $300 million of the drug this year.

Narcolepsy is a sleep disorder that causes excessive sleepiness, especially when the sufferer is engaging in monotonous activity.